The use of semipermeable membranes for the separation of gases or liquids in reverse osmosis or ultrafiltration processes is well known. For example, in a reverse osmosis process, high pressure saline water may be placed in contact with a semipermeable membrane which is permeable to water but relatively impermeable to salt. Concentrated brine and relatively pure water are separated thereby; the relatively pure water may then be utilized for personal use such as drinking, cooking, etc., while the brine may be discarded. In addition, membranes may also be utilized for the separation of various gases. The separation of a gas mixture utilizing a membrane is effected by passing a feed stream of the gas across the surface of the membrane. Inasmuch as the feed stream is at an elevated pressure relative to the effluent stream, a more permeable component of the mixture will pass through the membrane at a more rapid rate than will a less permeable component. Therefore, the permeate stream which passes through the membrane is enriched in the more permeable component while, conversely, the residue stream is enriched in the less permeable component of the feed.
This ability to separate gases from a mixture stream will find many applications in commercial uses. For example, gas separation systems may be used for oxygen enrichment of air, for improved combustion efficiencies and conservation of energy resources. Likewise, nitrogen enrichment of air may be applicable where inert atmospheres are required. Other applications for oxygen enriched gases may be improving selectivity and efficiency of chemical and metallurgical processes. Similarly, inert atmospheres such as may be provided for by this invention may also be utilized in chemical and metallurgical processes. Some other applications of gas separation would include helium recovery from natural gas, hydrogen enrichment in industrial process applications, and scrubbing of acid gases. Specific uses for oxygen enrichment of air would be breathing systems for submarines and other underwater stations, improved heart-lung machines, and other lung assist devices. Another specific application of a gas separation system would be an aircraft to provide oxygen enrichment for life-support systems and nitrogen enrichment for providing an inert atmosphere for fuel systems. In addition, gas separation systems may be used for environmental benefits, e.g., methane can be separated from carbon dioxide in waste gases for sewage treatment processes and oxygen enriched air can be produced to enhance sewage digestion.
Another use for which membranes may be employed is the separation of polysaccharides into useable constituents. For example, in many commercial enterprises sugar is utilized to a great extent for its sweetening properties. It is used in the sweetening of foods, for the manufacture of syrups and confectionary items, in preserves and jams as a chemical intermediate for detergents, emulsifying agents and other sucrose derivatives such as plasticizers, resins, glues, etc. The usual derivation of sugar is from cane sugar and sugar beets. It is obtained by crushing and extracting the sugar from the cane with water or extracting the sugar from the sugar beet with water followed by evaporation and purifying with lime, absorbent carbon and/or various liquids. The chief component of this type of sugar is sucrose, while other sugars may contain other polysaccharides such as dextrose and levulose (fructose). Other polysaccharides which possess sweetening properties include glucose, maltose, etc. The various polysaccharides possess varying degrees of sweetness, especially when in pure form and not contaminated by any reversion products.
One source of glucose which possesses a relatively high degree of sweetness and which, in turn, may be converted to fructose, the latter possessing an even greater degree of sweetness, is a starch. As is well known, starch is present in many naturally-occurring plants including corn, potatoes, rice, tapioca, wheat, etc. Heretofore, it has been known to treat starch with an enzyme such as amyloglucosidase to obtain glucose. However, the treatment heretofore provided entailed a relatively long residence time in order to obtain a glucose syrup which contained about 94% glucose. The relatively long residence time which has heretofore been required restricts the throughput of glucose and results in the appearance of reversion products which impart a bitter taste to the glucose, thus negating the sweetening property of the compound as well as requiring further treatment in order to remove the offending product. One such reversion product which imparts a bitter taste comprises isomaltose.
Many methods involving the use of an enzyme such as amyloglucosidase to convert starch into sugar have been tried. However, each of these methods has some disadvantages attached thereto. For example, when using a free enzyme, it is necessary to continuously replace the enzyme which is lost during the production of the desired saccharide. Likewise, when using an immobilized enzyme, the heretofore relatively long residence time has resulted in the appearance of unwanted side products.
One method of overcoming many of the disadvantages hereinbefore set forth is to contact the feedstock such as starch with an enzyme for a relatively short residence time and thereafter subjecting the partially hydrolyzed reaction mixture which is obtained from the conversion reaction to an ultrafiltration step wherein said reaction mixture is passed over a membrane whereby higher glucose syrup will pass through the membrane as a permeate while the retentate material containing unhydrolyzed oligosaccharides may be recycled for additional treatment.
As will hereinafter be shown in greater detail, by utilizing the membranes of the present invention, it is possible to obtain a high degree of saccharide separation, which results in the obtention of desired products at a relatively low operating cost.
Heretofore, membranes which may be used for reverse osmosis or ultrafiltration processes have been prepared using a wide variety of chemical compounds to obtain the desired membrane. For example, U.S. Pat. No. 3,892,655 discloses a membrane and a method for producing the membrane in which a thin polymer film is formed on the surface of a liquid, generally water, and is subsequently transferred to the surface of a porous supporting membrane. During the transfer of thin polymer film, the porous support is maintained in a wetted stage with the liquid. Another U.S. Patent, namely U.S. Pat. No. 3,526,588 discloses a macromolecular fractionation process and describes a porous ultrafiltration membrane which is selective on the basis of pore size. Likewise, U.S. Pat. No. 3,767,737 discloses a method for producing the casting of "ultra-thin" polymer membranes similar in nature to previously mentioned U.S. Pat. No. 3,892,655 in that the thin film of the membrane is formed on the surface of a liquid and transferred to the surface of a porous support membrane. However, the thin film poymer will thus inherently possess the same disadvantage which may be ascribed to the membrane formed by the latter patent in that the thin film of the finished membrane is weakly attached to the porous support and the membrane thus produced cannot withstand substantial back pressure when in operation.
As was previously mentioned, semipermeable membranes have been prepared from a variety of compounds by utilizing a polymer as the membrane-forming material. Examples of semipermeable membrane-forming polymers which have been used will include silicon-containing compounds such as dimethyl silicon, silicon-carbonate copolymers, fluorinated silicons, etc., polystyrene-polycarbonate, polyurethanes, styrenebutadiene copolymers, polyarylethers, epoxides, cellulose nitrate, ethyl cellulose, cellulose acetate mixed with cellulose esters, etc. The membrane resulting from the polymer is usually composited on a finely porous support membrane such as polysulfone, cellulose nitrate-cellulose acetate, etc., the composition being, if so desired, impregnated on a natural fabric such as canvas, cotton, linen, etc., or on a synthetic fabric such as Dacron, Nylon, Orlon, etc.
Examples of some semipermeable membranes which have been used in the past are those described in U.S. Pat. No. 4,005,012 which discloses a thin-film composite membrane comprising a cross-linked epiamine composited on a porous support such as polysulfone, the composition being impregnated on a backing material such as Dacron. U.S. Pat. No. 4,132,824 discloses an ultra-thin film of a polymer composite comprising a blend of a methylpentene polymer and an organopolysiloxane-polycarbonate interpolymer while U.S. Pat. No. 4,243,701 discloses a membrane comprising a dimethyl silicon composited on a cellulose nitrate-cellulose acetate support member.
In addition to the aforementioned patents, other U.S. patents also disclose membrane materials. For example, U.S. Pat. No. 4,262,041 discloses a process for preparing a composite amphoteric ion exchange membrane in which monomers having a primary to a quaternary ammonium radical such as vinylpyridine and monomeric units possessing alcohol or acid functions are cross-linked with a compound such as a diisocyanate by reaction of the isocyanate with the alcohol or acid group to obtain a cross-linked polymer. The use of compounds such as vinylpyridine in this patent is as a component of a copolymer, the other monomer component of said copolymer being a compound which is capable of receiving a cation exchange group to prepare the desired amphoteric copolymer. It is the intent of this patent to cross-link these copolymers in order to render them insoluble in the media in which they are to be used. As will hereinafter be shown in greater detail, the isocyanates which are present in our invention are not used for the same purpose as taught by this patent. In a similar manner, the heterocyclic nitrogen-containing compound which is used in our invention is not employed as one component of a charge-bearing, cross-linked copolymer, as taught by this patent.
U.S. Pat. No. 4,272,378 is drawn to a semipermeable membrane involving the use of polymers containing more than 40 mole percent of acrylonitrile, said acrylonitrile being copolymerized with other monomers. The result is a membrane which will possess characteristics and performances which are entirely different and apart from those which are possessed by the membranes of the present invention. U.S. Pat. No. 4,220,535 claims a multi-zoned hollow fiber permeator which may be obtained from any suitable synthetic or natural material suitable for fluid separation or as supports for materials which effect fluid separations. However, this patent does not disclose the interpenetrating polymer network which makes up the membrane of the present invention.
Another U.S. Patent, namely U.S. Pat. No. 3,951,789 discloses high diffusivity membranes which consist of physically admixed matrix materials with solutions of polyamideamines, said admixed solution being cast into membranes. In this patent, the matrix materials such as poly(phenyl ether) are intimately admixed with the polymers, this admixture being unlike the distinction from the membranes of the present invention which are hereinafter set forth in greater detail.
From a reading of the following specification and appended claims, it will be apparent that we have now discovered membranes which comprise an interpenetrating polymer network may be prepared by reacting an isocyanate-capped polyether and a heterocyclic nitrogen-containing compound, and compositing the resultant interpenetrating polymer network on a porous support backing material, said membranes possessing desirable characteristics when utilized in separation processes.